1. Field of the Invention
The invention relates generally to the field of signal telemetry for equipment used in drilling wellbores through the Earth. More particularly, the invention relates to methods and apparatus for locating faults in so-called “wired” drill pipe used for such telemetry.
2. Background Art
Devices are known in the art for making measurements of various drilling parameters and physical properties of Earth formations as a wellbore is drilled through such formations. The devices are known as measurement while drilling (“MWD”) for devices that measure various drilling parameters such as wellbore trajectory, stresses applied to the drill string and motion of the drill string. The devices are also known as logging while drilling (“LWD”) for devices that measure various physical properties of the formations, such as electrical resistivity, natural gamma radiation emission, acoustic velocity, bulk density and others. The various MWD and LWD devices are coupled near the bottom end of a “drill string,” which is an assembly of drill pipe segments and other drilling tools threadedly coupled end to end with a drill bit at the lowest end. During operation of the drill string, the drill string is suspended in the wellbore so that a portion of its weight is transferred to the drill bit, and the drill bit is rotated to drill through the Earth formations. Sensors on the various MWD and LWD devices can make the respective measurements during drilling operations. Wellbore drilling operators generally find that MWD and LWD measurements are particularly valuable when obtained during the actual drilling of the wellbore. For example, resistivity and gamma radiation measurements obtained during drilling may be compared with similar measurements made from a nearby wellbore so as to determine which Earth formations are believed to be penetrated by the wellbore at any moment in time. The wellbore operator may use such measurements to determine that the wellbore has been drilled to a particular depth necessary to conduct additional operations, such as running a casing or increasing the density of drilling fluid used in drilling operations. In general, MWD and LWD measurements may be communicated to the surface through telemetry between the bottom hole assembly and the surface. A telemetry device or tool in the bottom hole assembly with encode and transmit the data to the surface. It is often the case that the telemetry bandwidth cannot accommodate all of the MWD and LWD data that is collected. Thus, typically only a selected portion of the data is communicated to the surface, while all of the MWD and LWD data may be stored in one of the downhole components.
The signal telemetry that is most often used with MWD and LWD devices is so-called “mud pulse” telemetry. Mud pulse telemetry is generated by modulating the flow of the drilling fluid proximate the MWD or LWD devices in a manner to cause detectable changes in pressure and/or flow rate of the drilling fluid at the Earth's surface. The modulation is typically performed to represent binary digital words, using techniques such as Manchester code or phase shift keying. It is well known in the art that drilling fluid flow modulation is capable of transmitting at a rate of only a few bits per second. Thus, for most MWD and LWD applications, only a selected portion of the total amount of data being acquired is transmitted to the surface, while the data collected is stored in a recording device disposed in one or more of the MWD and LWD devices or in a another device for storing data.
Considerable effort has been made to provide a higher speed telemetry system for MWD and LWD devices. Such effort has been undertaken for a considerable time, and has resulted in a number of different approaches to high rate telemetry. For example, U.S. Pat. No. 4,126,848 issued to Denison discloses a drill string telemetry system, wherein an armored electrical cable (“wireline”) is used to transmit data from near the bottom of the wellbore to an intermediate position in the drill string, and a special drill string, having an insulated electrical conductor, is used to transmit the information from the intermediate position to the Earth's surface. Similarly, U.S. Pat. No. 3,957,118 issued to Barry, et al., discloses a cable system for wellbore telemetry. U.S. Pat. No. 3,807,502 issued to Heilhecker, et al., discloses methods for installing an electrical conductor in a drill string.
More recently, alternative forms of “wired” drill pipe have been described in U.S. Pat. No. 6,670,880 issued to Hall, et al. The system disclosed in the '880 patent is for transmitting data through a string of components disposed in a wellbore. In one aspect, the system includes first and second magnetically conductive, electrically insulating elements at both ends of each drill string component. Each element includes a first U-shaped trough with a bottom, first and second sides and an opening between the two sides. Electrically conducting coils are located in each trough. An electrical conductor connects the coils in each component. In operation, a time-varying current applied to a first coil in one component generates a time-varying magnetic field in the first magnetically conductive, electrically insulating element, which time-varying magnetic field is conducted to and thereby produces a time-varying magnetic field in the second magnetically conductive, electrically insulating element of a connected component, which magnetic field thereby generates a time-varying electrical current in the second coil in the connected component.
Another wired drill pipe telemetry system is disclosed in U.S. Pat. No. 7,096,961 issued to Clark, et al., and assigned to the assignee of the present invention. A wired drill pipe telemetry system disclosed in the '961 patent includes a surface computer; and a drill string telemetry link comprising a plurality of wired drill pipes each having a telemetry section, at least one of the plurality of wired drill pipes having a diagnostic module electrically coupling the telemetry section and wherein the diagnostic module includes a line interface adapted to interface with a wired drill pipe telemetry section; a transceiver adapted to communicate signals between the wired drill pipe telemetry section and the diagnostic module; and a controller operatively connected with the transceiver and adapted to control the transceiver.
The '961 patent describes a number of issues that must be addressed for the successful implementation of a wired drill pipe (“WDP”) telemetry system. For drilling operations in a typical wellbore, a large number of pipe segments are coupled end to end to form a pipe string extending from a Kelley (or top drive) located on a drilling unit at the Earth's surface and the various drilling, MWD and LWD devices in the wellbore with the drill bit at the end thereof. For example, a 15,000 ft (5472 m) wellbore will typically have about 500 drill pipe segment if each of the drill pipe segments is about 30 ft (9.14 m) long. The sheer number of pipe to pipe connections in such a WDP drill string raises concerns of reliability for the system. A commercially acceptable drilling system is expected to have a mean time between failure (“MTBF)” of about 500 hours or more. If any one of the electrical connections in the WDP drill string fails, then the entire WDP telemetry system fails. Therefore, where there are 500 WDP drill pipe segments in a 15,000 ft (5472 m) well, each WDP would have to have an MTBF of at least about 250,000 hr (28.5 yr) in order for the entire WDP system to have an MTBF of about 500 hr. This means that each WDP segment would have a failure rate of less than 4×10−6 per hour. Such a requirement is beyond the current state of WDP technology. Therefore, it is necessary that methods are available for testing the reliability of a WDP segment and drill string and for quickly identifying any failure.
Currently, there are few tests that can be performed to ensure WDP reliability. Before the WDP segments are brought onto the drilling unit, they may be visually inspected and the pin and box connections of the pipes may be tested for electrical continuity using test boxes. It is possible that two WDP sections may pass a continuity test individually, but they might fail when they are connected together. Such failures might, for example result from debris in the connection that damages the inductive coupler. Once the WDP segments are connected (e.g., made up into “stands”), visual inspection of the pin and box connections and testing of electrical continuity using test boxes will be difficult, if not impossible, on the drilling unit. This limits the utility of such methods for WDP inspection.
In addition, the WDP telemetry link may suffer from intermittent failures that would be difficult to identify. For example, if the failure is due to shock, downhole pressure, or downhole temperature, then the faulty WDP section might recover when conditions change as drilling is stopped, or as the drill string is tripped out of the hole. This would make it extremely difficult, if not impossible, to locate the faulty WDP section.
In view of the above problems, there continues to be a need for techniques and devices for performing diagnostics on and/or for monitoring the integrity of a WDP telemetry system.